Blakely and coworkers observed an epr signal when reaction mixtures containing 5' deoxyadenosylcobalamin (B12 coenzyme), an equivalent concentration of ribonucleotide reductase, dGTP (an allosteric activator) and dihydrolipoate (without the presence of the substrate) were incubated at 37 degrees and then freeze quenched. They propose that it may be B12r-deoxyadenosyl radical-enzyme complex. We propose to study the Mossbauer spectra of this intermediate species to help elucidate its structure. We require the finger prints of several cobalamins for comparison.